There is a well known basic conflict between efficient transonic flight of high-speed air vehicles and requirement for high-lift performance at subsonic speeds. Essentially there exist some basic differences in the aerodynamics associated with transonic and subsonic Mach numbers, and in the resulting pressure distributions on wing sections at the different flight regimes.
At transonic speeds, there is a general tendency for premature formation of shock waves on the upper surface of airfoils and for the development of shock-induced boundary layer separation. The resulting drag penalties and limitations on available lift (buffet and maximum lift limits) adversely affect the performance of the aircraft and constrain the allowable lift envelope. A classical solution for this problem generally adopted in the design and development of transonic cruise wings is a combination of wing sweepback and supercritical wing sections. Wing sweepback reduces the effective Mach number of transonic flight over the aerofoils, while the supercritical airfoil profiles prevent excessive flow acceleration on the upper surface thereof, producing favorable pressure distributions and reducing the wave drag penalties typically associated with subsonic aerofoils. Principles of supercritical design methodology are disclosed in NASA report Charles D. Harris, “NASA Supercritical Airfoils”, NASA Technical Paper 2969, 1990, the contents of which are incorporated herein in their entirety, and FIG. 5 thereof on page 41 illustrates graphically the qualitative differences on the flow and pressure distributions between subsonic and supercritical (transonic) aerofoils at transonic Mach Nos. of between 0.7 and 0.8.
As disclosed in the aforementioned NASA report, transonic pressure distributions over transonic aerofoils show fast flow acceleration at the leading edge of the airfoil, followed by the “roof-top” or plateau pressure distribution above the critical value of pressure coefficient (Cp*), followed by relatively weak shock wave and aft-loaded aft portion of the airfoil. Controlling the transonically shaped thickness distribution along the chord of the aerofoil section and the camber distribution allows the airfoil pressure distributions and location of shock wave for such transonic aerofoils to be controlled when designing a transonic aerofoil. Camber distributions for supercritical aerofoils are characterized in having very low camber (much less than 0.5% camber) at the forward portion of the airfoil, followed by a relatively higher cambered (around 1 or 2% typically), “cusped” aft portion that controls the lift of the airfoil). However, as is well known, this type of camber distribution, while being beneficial at transonic Mach numbers nevertheless generates a sharp suction peak at subsonic Mach numbers, which in turn triggers premature flow separation, particularly with increases in angle of attack, and limits the lift-carrying capabilities of supercritical airfoils at subsonic Mach numbers.
For civil aircraft applications, this basic incompatibility problem is solved by integration of sophisticated, leading and trailing edge high-lift devices in a wing that is designed for transonic cruise. Nevertheless, this solution is not always desirable, being mechanically complex, and carrying associated cost and maintenance elements, rendering it unsuitable for many applications, including, for example, UAV applications.
At subsonic Mach numbers, high values of maximum lift of single-element airfoils may be achieved for traditional subsonic aerofoil designs, which are characterized in having drooped and blunt leading edges, i.e., relatively large cambers, leading edge radii and wing thickness close to the leading edge. As is well known, at subsonic flight conditions, increased thickness and increased local radius at the forward portion of the airfoil generally prevent premature formation of a suction peak with increase in angle of attack and also delays flow separation from the upper surface of the airfoil, improving subsonic maximum lift. Typical examples of implementation of the concept of blunt leading edge are described in the following references: McGhee, R. J. and Beasley, W. D., “Low-Speed Aerodynamic Characteristics of a 17-Persent-Thick Medium-Speed Airfoil Designed for General Aviation Applications”, NACA Technical Paper 1786, 1980; Hicks, R. M. and Schairer, E. T., “Effects of Upper Surface Modification on the Aerodynamic Characteristics of the NACA 632-215 Airfoil Section”, NASA TM78503, 1979. However, high-lift subsonic airfoils are unsuitable and cannot operate at medium and high transonic Mach numbers because of premature formation of shock waves on their upper and lower surfaces, and the resulting fast deterioration of their lift-carrying capabilities with increasing Mach numbers.
Thus, the known groups of single-element, high-lift airfoils target a specific range of Mach numbers for their best lift performance, compromising on characteristics at off-design flight conditions. This is illustrated schematically in FIG. 1, showing typical lift domains of high-lift, low speed airfoils A1 and transonic aerofoils A2.